CD4+ helper T (TH) lymphocytes are essential organizers of adaptive immune responses and key mediators in immune-mediated inflammatory and allergic diseases. Upon activation by antigen-presenting cells (APC), naive TH cells undergo clonal expansion and functional differentiation into cytokine-secreting effector cells. Effector TH cells have been historically classified into TH1 and TH2 subsets. Recently, a novel TH subset, named TH17, that makes IL-17 has been identified by our and other groups. In the last funding cycle, we have investigated the molecular programs governing TH17 differentiation. We have identified key function of STAT3 in mediating cytokine-regulated TH17 differentiation. We further showed that STAT3 is required for upregulation of two TH17-specific orphan nuclear receptors RORa and RORg and that these two factors play synergistic and somewhat redundant function in TH17 differentiation. We also reported that RORa and RORg function in activating the transcription of IL-17 and IL-17F genes, two related cytokine genes but with differential in vivo function, and that the CNS2 element in IL-17-IL-17F gene locus enhances ROR-dependent transcription of IL-17 gene promoter in a reporter assay. Furthermore, we show that Foxp3+ Treg and TH17 cells exhibit antagonistic regulation yet with functional plasticity in vitro and in vivo. Smad2 and Smad3 molecules, both of which are activated by TGFb signaling, differentially regulate Treg and TH17 cell differentiation. Despite these achievements, there are still remaining questions about the transcriptional regulation in TH17 cells. First, are RORs important for maintaining cytokine transcription in mature TH17 cells? Second, how do RORs regulate the transcription of the IL-17 and IL-17F genes? Thirdly, considering RORs associate with various co-activators for their function, what co-activators are required for TH17 cell differentiation? We will pursue these questions in the proposed grant. First, we will determine the function of RORg and RORa in mature TH17 cells and in Treg cells. Secondly, we will examine how RORs regulate TH17 cytokine transcription. We will identify additional elements that may be also required for IL-17F transcription. Lastly, we will genetically analyze the functio of SRC molecules in TH17 differentiation. We will investigate their function and regulation during TH17 cell differentiation in vitro and in vivo. Overall, the proposed project will center on the transcription factors that were shown in the previous cycle to be crucial in TH17 differentiation. We will analyze the biological function of these factors as well as the mechanisms underlying their function. These studies will further substantiate our understanding on these important regulators and facilitate ongoing efforts in targeting these pathways in treatment of human diseases.